The invention relates to a process for the pretreatment of material surfaces and the use of these passivated materials in the preparation or processing of polycarbonates.
By means of this process, it is possible to minimize the interaction between polycarbonate and metal and, in the processing and synthesis of polycarbonate, to obtain a high-quality polymer which in particular does not become discoloured and is free of insoluble constituents.
The polycarbonate synthesis can be carried out continuously or batchwise. The reaction can therefore be effected in stirred vessels, tubular reactors, pumped-circulation reactors or stirred vessel cascades or combinations thereof. Typical materials for these stirred vessels or tubular reactors and pipelines, etc., are, for example, V4A steel 1.4571 or Hastelloy-C.
The washing of the polycarbonate solution is effected by passages through settling vessels, stirred vessels, coalescers or separators or combinations of these measures. These plant components likewise consist of the abovementioned materials.
The isolation of the polymer from the solution can be effected by evaporation of the solvent by means of temperature, vacuum or a heated entraining gas. Other isolation methods are crystallization and precipitation. These process steps, too, are carried out in containers comprising the abovementioned media. In a thermal work-up process, the material used is of particular importance owing to the higher temperature. Such a thermal work-up process is, for example, removal of the solvent by distillation by superheating and depressurization, a so-called “flash process” (also see “Thermische Trennverfahren [Thermal separation processes]” [VCH Verlagsanstalt 1988, page 114). All these processes are described in the patent literature and in text books and are familiar to the person skilled in the art. On removal of the solvent by means of temperature (distilling off) and the technically more effective flash process, highly concentrated polymer melts are obtained. The residues of the solvent can be removed from the highly concentrated polymer melts thus obtained, either directly from the melt by means of vented extruders (BE-A 866 991, EP-A 0 411 510, U.S. Pat. No. 4,980,105, DE-A 33 32 065), thin-film evaporators (EP-A 0 267 025), falling-film evaporators, strand evaporators or by friction compacting (EP-A 0 460 450), optionally also with addition of an entraining agent, such as nitrogen or carbon dioxide, or with the use of a vacuum (EP-A 0 039 96, EP-A 0 256 003, U.S. Pat. No. 4,423,207). Here too, the material used, which is in contact with the polymer melt, has a special role.
The polycarbonate prepared with the aid of this process is suitable in particular for high-quality injection moulded articles, in particular those in which high transparency and low YI (yellowness index) are important, such as, for example, optical data stores, diffuser screens or panes for the automotive sector generally.
It is known that, during processing and synthesis of polycarbonate, the contact with material surfaces, in particular at elevated temperatures, for long residence times and with use of iron-containing materials, damage to the polycarbonate is caused. The damage manifests itself in a deterioration of the colour (yellowing) and the formation of insoluble fractions. This damage is particularly evident on contact with fresh surfaces of stainless steels, for example in start-up processes. The damage, in particular yellowing, of polycarbonate in the presence of steel surfaces or metal ions has been comprehensively described before, for example in EP0819717, EP0905184, EP0386616.
The prior art comprises various approaches for minimizing this polycarbonate damage.
WO2002037148, EP1383821, EP0512223, EP1155737 and EP0635532 describe the use of high-quality materials having a low iron content. Owing to the high costs for the procurement and processing of particularly low-iron alloys, the wide use of such high-quality materials is not an economically expedient solution. The material composition stated in EP1155737 and having nickel contents of 5-15% and chromium contents of 10-20% is moreover not adequate for keeping the polycarbonate damage in the desired low range under typical synthesis and processing conditions.
EP0905184 describes the use of phosphite diesters as stabilizing additives to the polycarbonate, which suppress the interaction with metal ions. The metering of a stabilizer is generally undesired owing to the change in the product composition or because of an effect on the reaction.
US20080154018, US20080210343, EP1156071, EP0819717 describe treatment methods for making metallic materials inert to polycarbonate. The methods described in US20080154018, US20080210343, EP1156071 and EP0819717 are very complicated and the result is inadequate. In EP1156071 and EP0819717, various wash processes are combined with thermal treatment of the metal surface. US20080154018 and US20080210343 describe thermal and thermal oxidative treatment steps. In addition, the object in US20080154018 and US20020210343 was to reduce the formation of fluorescent particles, while the person skilled in the art finds no indication in these laid-open-applications as to how he can enhance colour and insoluble residues.
The use of phosphate for improving the corrosion resistance is known and is described, for example, in CH-580685, JP-81018675, DE-4129529, DE-19621184, U.S. Pat. No. 6,117,251, C. A. Melendres et al., Electrochimica Acta 34 (1989) 281. As a rule, aqueous phosphate or phosphoric acid solutions are used for the treatment of the metal surfaces with the aim of improving the resistance of the metal surface to corrosion. That such a treatment of the surfaces may lead to an improved polycarbonate quality is however not mentioned.
It is therefore the object of the invention to develop a simple method for the pretreatment of steel surfaces which minimizes the interaction of the metal surface with polycarbonate and thus reduces or as far as possible suppresses the damage. Ideally, this method should also be capable of being used directly in operation on existing apparatuses and machines, not only during production or before the assembly of new apparatuses.